State-of-the-Art Quantum Chemistry Meets Variable Reaction Coordinate Transition State Theory to Solve the Puzzling Case of the H2S + Cl System

TL;DR

This study combines advanced quantum chemistry and variable reaction coordinate transition state theory to accurately determine the reaction rate of H₂S with Cl, resolving previous discrepancies and demonstrating the reliability of transition state theory with high-level computations.

Contribution

It introduces a highly accurate computational approach that combines state-of-the-art quantum chemistry with variable reaction coordinate transition state theory to resolve previous uncertainties in reaction kinetics.

Findings

Reaction rate constant at 300 K matches experimental data

No failure of transition state theory needed with accurate computations

Provides a robust method for atmospheric and astrophysical reaction analysis

Abstract

The atmospheric reaction of H$_2$S with Cl has been reinvestigated to check if, as previously suggested, only explicit dynamical computations can lead to an accurate evaluation of the reaction rate because of strong recrossing effects and the breakdown of the variational extension of transition state theory. For this reason, the corresponding potential energy surface has been thoroughly investigated, thus leading to an accurate characterization of all stationary points, whose energetics has been computed at the state of the art. To this end, coupled-cluster theory including up to quadruple excitations has been employed, together with the extrapolation to the complete basis set limit and also incorporating core-valence correlation, spin-orbit, and scalar relativistic effects as well as diagonal Born-Oppenheimer corrections. This highly accurate composite scheme has also been paralleled by less expensive yet promising computational approaches. Moving to kinetics, variational transition state theory and its variable reaction coordinate extension for barrierless steps have been exploited, thus obtaining a reaction rate constant (8.16 x 10$^{-11}$ cm$^3$ molecule$^{-1}$ s$^{-1}$ at 300 K and 1 atm) in remarkable agreement with the experimental counterpart. Therefore, contrary to previous claims, there is no need to invoke any failure of the transition state theory, provided that sufficiently accurate quantum-chemical computations are performed. The investigation of the puzzling case of the H$_2$S + Cl system allowed us to present a robust approach for disclosing the thermochemistry and kinetics of reactions of atmospheric and astrophysical interest.